This also implies to me that consciousness, not being a physical thing itself, comes and goes within the brain, with the sleep cycle or just through lack of attention. The only interaction between one instance of consciousness and the following one would then be via memories.

Assuming you mean cognition, this is essentially the opposite of the idea of panexperientialism, or at least not compatible.

Thus, a single atom cannot be conscious. It could at a stretch be regarded as a miniature body, perhaps, but could it contain a memory system? Presumably not.

The universe is one giant state machine with the memory of it's current state and it's current state deciding what to invoke next.

I believe the word for that is idealism.

https://en.wikipedia.org/wiki/Idealism

https://plato.stanford.edu/entries/berkeley/

A difficulty is that it may interact with questions about the nature of existence and the nature of time. Would consciousness work properly in a deterministic block universe? What would be the relationship between my consciousness now and my consciousness of 10 seconds ago, do they both exist independently? Could you have consciousness that worked in a spacial dimension instead of time, by writing out the relevant calculations on a sheet of paper?

that particular atom exists because it feels like something to be that particular atom

seems to beg the question somewhat - where is "feeling like something" coming from?

Everytime I think about this, I end up with "the fact that something is not in the state it desires to be in seems sufficient and necessary".

Maybe electrochemical imbalance is what's experienced as consciousness, all the way down to the smallest particles (or even beyond) - then, organic consciousness would just be an immense abstraction of the same thing that drives physics itself.

I feel like this is satisfying, but then again, I'm not Joscha Bach :-)